Hydraulic pumps



G. L. HARRISON HYDRAULIC PUMPS Nov. 24, 1959 2 Sheets-Sheet 1 Filed June25, 1958 Fla.

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3244M h-romsvs NW; 9 G. L. HARRISON 2913985 HYDRAULIC PUMPS filed June1958 2 Sheets-Sheet 2 Geoffre 'L. fl l r zn W i w A-rmaueva UnitedStates Patent HYDRAULIC PUMPS Geoifrey L. Harrison, Scarborough,Ontario, Canada, as-

signor to Dowty Equipment of Canada Limited, Ajax, Ontario, Canada, aCanadian company Application June 25, 1958, Serial No. 744,537 Claimspriority, application Great Britain June 25, 1957 r 3 Claims. (Cl.103-37) This invention relates to hydraulic pumps which are intended togenerate hydraulic pressure of a selected value and to deliver hydraulicliquid until such pressure is attained, after which the volume ofdelivered liquid will drop to a value merely suflicient to maintain theselected pressure. The simplest way to accomplish this result is toprovide an ordinary fixed displacement pump with a bypass or reliefvalve having an adjustable loading spring whereby the valve will openwhen the liquid pressure is attained. With such a simple arrangement,when the desired pressure is attained the fulljpump output is by passedat pressure and in consequence the prime mover-driving the pump is stillrequired to do a' considerable amount of work, all of which is convertedinto heat.

'The'object of the present invention is to provide a very simplehydraulic pump which s capable of delivering liquid up to a selectablehydraulic pressure and in which, the rate of displacement of liquid isconsiderably reduced when this pressure is attained so that thehydraulic liquid is not unduly heated whilst maintaining the pressure.

In accordance with the present invention a hydraulic pump comprises alarge and a small piston located in large and small cylinders andreciprocated by rotation of a single shaft, to deliver pressure liquidto a hydraulic cutout, a spring loaded valve with adjustable springloading being provided which is sensitive to the hydraulic outputpressure and which is moved by such pressure when it reaches a valuedetermined by the said spring loading to connect the output of the largepiston and cylinder to a zone of low pressure. The spring loaded valvealso opens a by-pass from the output of the small piston and cylinder ata slightly higher pressure than that at which the large piston andcylinder is by-passed.

One example of the invention will now be described with reference to theaccompanying drawings in which Figures 1 and 2 are cross-sectionsthrough the pump taken perpendicularly to one another on the respectivesection lines 11 and 2-2.

The example of the invention is for use as a hydraulic pressuregenerator in a hay baling apparatus. The complete pump is enclosed in ahydraulic oil reservoir 1, a single shaft 2 extending for connectionindirectly to the prime mover operating the hay baler. Inside thereservoir 1 a cylinder block 3 is located which extends to the shaft 2and in which bearings 4 for the shaft are located. The shaft carries aneccentric 5 around which an eccentric ring 6 is located. This ring isconnected pivotally at 7 to one end of a large piston 8. The largepiston 8 reciprocates in a large cylinder 9 formed in the cylinder block3. From the opposite end of the large piston a smaller diameter piston11 extends, coaxially formed and located in a smaller diameter cylinder12. A port respectively 13 and 14 is formed in the wall of each cylinderadjacent to the position of the included piston when it is withdrawn toits greatest extent. These ports form inlets into the cylinders. At

the end of each cylinder remote from the eccentric, an output portrespectively 15 and 16 is located, a non-return valve respectively 17and 18 being included in each port. These two output ports are connectedthrough pipe 19 to a hydraulic pressure connection 21 located on thewall of the reservoir 1.

Also in the cylinder block a piston valve is located comprising a longsmall diameter cylinder 22 bored in the cylinder block which contains asimple piston valve member 23 having a single groove 24 formed centrallythereof. One end of the piston valve member projects from the block andis engaged by a helical compression spring 25 which extends towards theupper part of the reservoir. The spring is there connected to a screwmechanism 26 which is adjustable by a manually-rotated knob 20 arrangedoutside the reservoir. A pair of ports 27 and 28 enter the long cylinder22 at the position of the groove in the piston valve member, port 27being in connection directly to the output 15 of the large cylinder andthe port 28 being connected directly to the reservoir. The arrangementnormally is that outlet 15 from the large cylinder 9 feeds into thegroove 24 whilst the piston valve member 23 closes the port 28 leadingback to the reservoir. The end of the piston valve member 23 opposite tothe adjustable loading spring 25 is exposed through passage 29 to theoutput pressure as delivered at the hydraulic output connection of thepump, and this pressure will normally act to urge the piston valvemember 23 to compress the adjustable loading spring 25. A further port31 is provided to begin to vent the hydraulic output pressure back toreservoir when the piston valve member 23 is moved a sufiicient distanceagainst spring compression to cause full bypass of the output of thelarge cylinder back to reservoir through port 28.

When in operation on a hay baler the pressure connection 21 on the wallof the reservoir 1 is connected via a single line to a displacement-typehydraulic ram. This ram is connected to a pair of rails which maintainpressure against two sides of a bale of hay or straw. The side pressureof the rails against the hay or straw needs to be adjustable by theoperator in accordance with the weight of bale required, and thecondition of the straw or hay at time of baling. The adjustment is madeby the operator turning the knob 20 on the reservoir 1 and observing thepressure shown on the gauge 32 provided. During initial movement thewhole output of both cylinders in the pump is fed into the jack to movethe rails to an appropriate position. When the pre-selected pressure, asdetermined by the setting of the knob 20, is reached the valve 23 movesand the output of the large cylinder 9 is fully by-passed into thereservoir whilst the output of the small cylinder 12 is lay-passedwholly or partially through port 31 to maintain the desired pressure inthe jack. The amount of oil that is pumped at pressure back to reservoir1 is therefore only the output of the small piston 11 and cylinder 12and the heat developed is comparatively small. When an increase inpressure is required the whole output of both cylinders is available formoving the jack as quickly as possible to its new position. The systemis in balance when the outward force exerted by the bale of hay or strawequals the inward force exerted by the hydraulic ram. If a change in theoutward force exerted by the bale occurs the balance is upset and thehydraulic ram extends or contracts until the system is again in balance.If the outward force exerted by the bale decreases, the pump pressuredrops momentarily, thereby closing the pump by-pass passage or passages28 and 31, and oil flows from .the pump to the ram until the by-passpressure pre-selected by the operator is restored. If the outward forceexerted by the bale increases, the pump pressure rises momentarily,thereby increasing the opening of the pump by-pass 31 and causing oil toflow from the ram to the reservoir until the by-pass pressurepre-selected by the operator is restored. The ram is thereforecomparable to a spring of zero rate. This ensures that if the length andthe height of the bale is fixed the width of the bale may vary withoutany appreciable change in the outward forces exerted by the bale. Thisallows the volume of the bale to change and compensate for localvariations of hay or straw density, thereby ensuring that bale weightremains substantially constant.

I claim as my invention:

1. A hydraulic pump comprising a large piston and cylinder unit and asmall piston and cylinder unit, means to reciprocate said pistonsconjointly, an output port from each cylinder and a non-return valve ineach thereof, a common hydraulic output passage with which the saidoutput ports communicate, past their non-return valves, input ports tothe respective cylinders from a low pressure hydraulic supply source,closed by pressure-inducing movement of their pistons; a spring-loadedby-pass valve comprising an elongated plunger grooved intermediatebetween its ends and a by-pass valve cylinder wherein said plunger isslidably mounted, spring means urging said plunger in one sense, a firstby-pass passage between said valve cylinder, in the vicinity of theplungers groove, and the interior of the large cylinder, and a secondby-pass passage between said valve cylinder, at the end towards whichthe plunger moves under the influence of its spring means, and thecommon outlet passage; a first relief port spaced in the sense ofpressure-induced movement of the plunger from said first by-pass passageand affording communication between the latter, and hence the firstcylinder, and a low pressure region with increasing pump outputpressure, and a second relief port adjacent that end of the plungerwhich is subjected to common output pressure, and spaced in the samesense to connect the common output passage at least partially to a lowpressure region following opening of the said first relief port.

2. A hydraulic pump as set forth in claim 1, including means to adjustthe spring means for opening of the first relief port fully uponattainment of a predetermined output pressure, and for further openingthe second relief port upon the output pressure tending to exceed suchpredetermined pressure.

3. A hydraulic pump as claimed in claim 1 including a reservoirsurrounding the pump and by-pass valve and also enclosing a quantity ofhydraulic liquid at low pressure to feed the large and small piston andcylinder units and to collect liquid from the by-pass valve.

References Cited in the file of this patent UNITED STATES PATENTS

